Electric motors, conventional manufacturing method, consists on a stator with unipolar coils (N or S magnetic pole) in alternate magnetic pole array. Rotor could be manufactured with same unipolar coils or permanent magnets.
Classic manufacturing method consists in winding or staking wires or ribbons of electrical conductive material around a core.
The electric motors having stator constructed with classic coils and rotor constructed with permanent magnets have an efficiency of 85-95%. It is normal because approx. 50% of the magnetic fields is provided by the permanent magnets.
The electric motors who have stator and rotor constructed with classic coils have an efficiency of 35-43%. Conclusion:
Low efficiency of the coil classic manufacturing method: due to
More than 50% of magnetic field generated by electrical current is lost because:
Generated outside of the coil and do not magnetize any core.
Important length of electrical conductor material (wires) it used in turns and grows with the number of layers or windings strands.
Only a small fraction of the magnetic field is perpendicular to the core surface (especially in the case when round wire section is used).
Air gaps between wires/stands/core.
Proximity effect (Induced currents generated by the magnetic field of adjacent wire)
Otherwise, this manufacturing method has other disadvantages:
Impossible to realize a single dipolar coil with its both magnetic poles (N/S) on wished side of the coil.
No magnetic field amplificatory factor (applied only on dipolar coil with external cores).
Impossible to manufacture complex shapes (obligatory to have a constant section in case of ribbon winding).
Cogging run motors.
Less mechanical rigidity.
Slowly winding process in reason of small space between cores and insulation injured risk.